A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) is designed to measure the concentration distribution and the relaxation time distribution of nuclear spins at a desired portion to be examined within an object to be examined, utilizing the nuclear magnetic resonance phenomenon, and to display an image of a cross section of the object obtained from the obtained data.
The nuclear spins of an object, which is placed within an apparatus for generating a uniform and strong magnetic field, perform precession at a frequency (Lamor frequency) determined by the magnetic field strength around an axis in the direction of the magnetic field. When a radio-frequency pulse having a frequency equivalent to the Larmor frequency is irradiated to the object from the outside, the nuclear spins are excited and shift to a higher energy state (nuclear magnetic resonance phenomenon). When the radio-frequency pulse irradiation to the object is terminated, the nuclear spins return to a low energy state at a time constant in accordance with their respective states, and they radiate electromagnetic waves (NMR signal) to the outside of the object. The electromagnetic waves are detected by a radio-frequency receiving coil, which is synchronized with their frequency.
Here, gradient magnetic fields on x-, y-, and z-axes are applied to the magnetic field space so as to add positional information to the NMR signals. Consequently, the positional information within the space can be obtained as frequency information.
For radio-frequency pulse irradiation, an irradiating coil is used which generates a radio-frequency magnetic field in a direction perpendicular to the static magnetic field direction. This irradiating coil has been researched and modified to obtain an improvement of the irradiation uniformity over a wide area in the magnetic field space, and various types of coils have been used.
FIG. 1 is a diagram showing one example of an irradiating coil, wherein an example of a planar birdcage coil is shown. Referring to FIG. 1, two concentric ring conductors 1a and 1b having different sizes on the same plane are mutually connected by a plurality of line conductors 2, extending in the radial direction.
FIG. 2 is an equivalent circuit diagram of the planar birdcage coil. Referring to FIG. 2, a loop a and a loop b designate the ring conductor 1a and the ring conductor 2a, respectively. The loop a and the loop b are usually synchronized at a magnetic resonance frequency. For the synchronization, condensers C and coils L are used.
FIG. 3 is a diagram showing the voltage distribution and current distribution in the loop a shown in FIG. 2. Referring to FIG. 3, the current is maximized and the voltage is minimized at the feeding points d, since the loop a is synchronized at the resonance frequency.
FIG. 4 is a diagram showing one example of the irradiating planar birdcage coil shown in FIG. 1, which is mounted in an MRI apparatus. Referring to FIG. 4, an irradiating coil 18 is usually located in the vicinity of an object 14 to be examined so as to efficiently apply an irradiation pulse to the object 14. Around the object 14, there are arranged a receiving coil 17 for receiving magnetic resonance signals that are generated from the object 14, a pre-amplifier 22 for amplifying the received signals, distribution lines 23 for connecting the signals that are amplified by the pre-amplifier 22 to an A/D converter (not shown), and the like, which are usually mounted in a bed on which the object 14 is supported. The receiving coil 17, the pre-amplifier 22, and the distribution lines 23 may be arranged in one location, for ease of manufacturing and operationality.
FIG. 5(B) is a graph showing an intensity distribution of radiation that is applied to the object, the intensity having been measured at arbitrary points (c, d, e, f, g, h, i, and j, along the phantom line circle in FIG. 5(A)) on the line conductors 2 of the above-mentioned planar birdcage coil. In FIG. 5(B), the vertical axis represents the irradiation intensity, and the horizontal axis represents locations along the phantom line circle. As shown in FIG. 5(B), the irradiation intensity is larger on the line conductors 2 than in the space between the line conductors 2, and it has a pulse shape that is pulsing with a uniform width.
As shown in FIG. 4, in the MRI apparatus, imaging is performed while the object 14, on which the receiving coil 17 is placed, is mounted on the bed. The pre-amplifier 22, for amplifying the signals sent from the receiving coil 17, and the distribution lines 23 are installed in the bed.
To adjust the location of the object 14 so as to image a particular portion, the bed is constructed so that it can be moved. However, the positional relation between the pre-amplifier 22, the distribution lines 23 and the irradiating coil 18 is changed by moving the bed, since the pre-amplifier 22 and the distribution lines 23 are installed in the bed. Consequently, a potential difference is generated between the potential of the receiving coil 17, the pre-amplifier 22, and the distribution lines 23, and that of the irradiating coil 18, and so a radio-frequency coupling is generated. The strength of the radio-frequency coupling fluctuates when the positional relation is changed, since the positional relation affects the potential difference.
Next, the fluctuation of the radio-frequency coupling will be described with reference to FIG. 6. Referring to FIG. 6, the irradiating coil 18 operates as a QD coil. Here, reference number 8 represents a condenser C in the circuit diagram of FIG. 2, and it is located where the voltage is maximized in FIG. 3.
Usually, in a QD coil, one feeding point does not affect a resonant circuit of another feeding point, since the two feeding points are perpendicularly arranged. However, as shown in FIG. 4, when only one side of the irradiating coil is subject to radio-frequency coupling due to the effect of the distribution lines 23 (in FIG. 6, the pre-amplifier 22 and the distribution lines 23 are arranged as shown) or the like, the pre-amplifier 22 and the distribution lines 23 are located where the potential of the irradiating coil 18 is high. In this case, radio-frequency coupling between the pre-amplifier 22, the distribution lines 23, and the irradiating coil 18 is caused to fluctuate with fluctuation of the positional relation between the pre-amplifier 22 and the distribution lines 23.
Next, this fluctuation of the potential will be described with reference to FIG. 7. FIG. 7 shows a voltage that is generated in resonant circuits d and d′ in FIG. 6. The respective voltage distributions of circuits d and d′ are indicated by the v1 and v1′ solid lines in ideal conditions. The potential of v1′ at a point where the potential of v1 is highest (or lowest) is the base potential, and this prevents radio-frequency coupling. However, when the object 14 is moved, or the bed is moved, the pre-amplifier 22 and the distribution lines 23 are moved relative to the irradiating coil 18. They are moved to a location where the potential of the resonant circuit d is the highest, whereby the potential is shifted as shown in FIG. 7 (represented by the broken line). Consequently, the location where the potential of d is a base potential is shifted from the point e, whereby fluctuation of a potential difference between d and d′ is generated. Consequently, the orthogonality of the QD coils d and d′ between d and d′ is lost, and a radio-frequency coupling is generated between them.
An error in the phase of the radio-frequency magnetic field generated from the two coils of the QD coil occurs, varying from 90 degrees due to the above-described radio-frequency coupling, and, consequently, the uniformity of the radiation pulse and the irradiation efficiency are deteriorated. However, in the conventional technique, consideration for the radio-frequency coupling between the irradiating coil 18 and the distribution lines 23 in the vicinity thereof has not been given.
Also, in the planar birdcage coil, line conductors 2 are radially arranged between the two ring conductors 1a and 1b, which have different sizes. Consequently, the irradiation intensity is large in the vicinity of the central portion close to the ring conductor 1b, since the distance between two line conductors is narrow. On the contrary, the irradiation intensity is small on the periphery close to the ring conductor 1a, since the distance between two line conductors is broad.
That is, the irradiation intensity distribution is lower where the line conductors 2 are farther from the ring conductor 1b, whereby the overall irradiation distribution is not uniform. Therefore, when imaging a relatively large object, the sensitivity in the portions other than the central portion in the obtained image is lower, in comparison with the central portion.
Thus, in the conventional technique, consideration to provide a uniform irradiation pulse has not been given.